Polylactic acid (referred to herein as “PLA”) is becoming an important industrial chemical because it is a biodegradable plastic that is not derived from petroleum. PLA is a renewable resource that is derived from corn, potatoes, and various plants. PLA is referred to as a carbon circulation-type plastic because it is produced from lactic acid and after use can be broken down to water and carbon dioxide through biodegradation or incineration.
PLA is transparent, has a mechanical strength at room temperature that is close to that of polyethylene terephthalate (PET), and is easily manipulated. Because of these characteristics, PLA is expected to become a general-purpose plastic material that is commonly used in daily life.
PLA does however have drawbacks based on its heat resistance, fragility, and low flexibility. Enhancing the flexibility of PLA has received much attention, and numerous methods for improving that characteristic have been proposed.
In one method of improving the flexibility, other aliphatic ester, ether or carbonate components are introduced into the polylactic acid skeleton by copolymerization in order to impart more flexibility. This method increases the cost of the resulting product because of the nature and amount of the added components.
Another method of improving the flexibility adds a plasticizer having a low molecular weight (for example, polyethylene glycol) to the PLA. However, addition of a plasticizer causes bleeding (separation) of the plasticizer from the surface, which can result in a sticky, tacky surface.
A further method adds an acrylic acid ester resin having a relatively low glass transition temperature (Tg) to the PLA (see J. L. Eguiburu et. al., Polymer, Vol. 39, No. 26, page 589 (1998)). Similarly, Japanese Unexamined Patent Publication (Kokai) No. 2003-286401 (JP '401) describes a PLA-containing resin composition comprising polylactic acid and a second polymer mainly comprising an unsaturated alkyl carboxylate-based unit that has a glass transition temperature of 10° C. or less. The weight average molecular weight of the second polymer is 30,000 g/mole or less. Japanese Unexamined Patent Publication (Kokai) No. 2004-10842 (JP '842) describes a PLA resin composition comprising (a) a PLA and (b) an acrylic acid ester-based oligomer having a constitutional unit represented by:
wherein R1 represents an alkyl group having a carbon number of 1 to 3.
These methods provide typically do not provide a PLA resin with the desired combination of flexibility and elongation properties. Further, in some of the known compositions where a second polymeric or oligomeric material is added to the PLA and the compositions are held room temperature for several days, the second polymeric or oligomeric material can separate from the composition resulting in a sticky texture that is not useful.
Therefore, there remains a need for further methods of enhancing at least one property of a PLA composition.